1. Field of the Invention:
The present invention relates to an apparatus and method for displaying an image through modulation of an incident light from a light source by a spatial light modulator which modulates the light at each pixel thereof in a binary manner.
2. Description of Related Art:
Liquid crystal display units using a liquid crystal panel as a spatial light modulator have widely been used as image displaying apparatuses which display an image through modulation of an incident light from a light source by the spatial light modulator which modulates the light at each pixel thereof. Many of such conventional image displaying apparatuses are of a type in which a TN liquid crystal or an STN liquid crystal is used as the liquid crystal panel and continuously changed in state to modulate the light intensity. However, such liquid crystal panels responds slowly and cannot operate at a high speed.
To solve such problems of the conventional liquid crystal panels, a spatial light modulator has been proposed which is made of a light modulating material capable of working fast, such as ferroelectric liquid crystal (FLC). However, the light modulating material such as the FLC is hard to continuously change in state and can normally take only two states. Therefore, the light or optical modulation by the spatial light modulator using such a light modulating material only turns on and off a light for the binary light modulation.
For a display with a gradation of light intensity in an image displaying apparatus using such a spatial light modulator, a pulse width modulation is done by the spatial light modulator turning on and off the incident light. The human eyes have a persistence so that a quantity of incident light upon the eyes is integrated and the result of the integration is recognized as a light intensity. So, if the pulse width modulation could be effected at a sufficiently high speed, the human eyes would recognize an incident light as if the light had a gradation of intensity.
FIG. 1 shows the concept of such an image displaying apparatus. A light source 101 irradiates a light through a light-irradiation optical system 102 to a spatial light modulator 103. The light reflected from the spatial light modulator 103 is projected by a light-projection optical system 104 onto a screen 105. Thus an image is displayed on the screen 105. The light source 101 is continuously turned on to provide the light at a predetermined intensity, and the light from the source 101 is modulated in pulse width by the spatial light modulator 103 which turns on and off the light source 101. It should be appreciated that the spatial light modulator 103 may be of a transmission type although that illustrated in FIG. 1 is of a reflection type.
FIG. 2 shows the basic principle of a pulse width modulation adopted in the above-mentioned image displaying apparatus to realize a display with a gradation of light intensity. FIG. 2 shows a relationship between patterns of modulation by the spatial light modulator 103 and light intensities recognizable by the human eyes (recognizable intensity). As illustrated, the human eyes will integrate a quantity of light reflected and modulated by the spatial light modulator 103, and recognize the integrated value as an intensity. Therefore, even if an actual light intensity is constant, as the width of a light pulse reflected from the spatial light modulator 103 is changed, the intensity recognized by the human eyes will change correspondingly to a magnitude of the pulse width change. Therefore, by controlling the pattern of modulation by the spatial light modulator 103, it is possible to effect an intensity modulation of a light.
As illustrated in FIG. 3A, however, if a characteristic (property) A in an area in the plane of the spatial light modulator 103 is different from a characteristic (property) B in another area, namely, if there exists an in-plane variation in on/off characteristic of the spatial light modulator 103, the intensity response of a light modulated by the spatial light modulator 103 will vary from one to another area with a result that an intensity recognized by the human eyes will vary. More particularly, if the spatial light modulator 103 varies in in-plane characteristic from one to another area, the light pulse intensity and shape, premises for intensity modulation through the pulse width modulation, will also vary from one to another in-plane area, so that the intensity will be nonuniform.
This problem can be solved with a completely uniform characteristic over the plane of the spatial light modulator 103. However, it is extremely difficult to have the complete uniformity of the characteristic over the plane of the spatial light modulator 103. Thus, it has been difficult with the conventional image displaying apparatus to eliminate the light intensity nonuniformity due to the nonuniform in-plane distribution of the characteristic of the spatial light modulator 103.
For a pulse width modulation for a limited period with an increased number of intensity levels, the minimum pulse width has to be reduced. In an ordinary image displaying apparatus, for example, the display period of one screen is about 16 msec for which a pulse width modulation should be done to realize a display with a gradation of light intensity. Under an assumption that a pulse width modulation is done for the period of 16 msec, if an intensity data is of 8 bits and has 256 intensity levels, the necessary minimum pulse width has to be 62 .mu.sec. In case an intensity data is of 10 bits and has 1024 intensity levels, the minimum pulse width has to be 15 .mu.sec.
More particularly, for display of ail image with a gradation of light intensity by a pulse width, modulation, the minimum pulse width should be several tens .mu.sec. Since the TN liquid crystal and STN liquid crystal have a response speed of several msec to several hundreds .mu.sec, the minimum pulse width cannot be several tens .mu.sec. On the contrary, the light modulating material, such as FLC, can attain a minimum pulse width of several tens .mu.sec. However, even if an light modulating material having a high response such as FLC is used, it is necessary to use a very high voltage to excite the light modulating material in order to have such a small minimum pulse width. Namely, the requirements for excitation of the light modulating material are very difficult to meet. Therefore, a pulse width modulation in the conventional image displaying apparatus using a spatial light modulator which provides a binary modulation of a light cannot provide a satisfactory display of an image with a gradation of light intensity.